Unruptured Intracranial Aneurysms (UIA) constitute a significant public health problem in the United States, which is growing in magnitude. The economic and social implications of optimizing clinical practice in this area are striking, given the considerable and escalating frequency with which UIAs are now detected in the population. The prevention of unnecessary death and disability related to UIA depends to a large degree upon a better understanding of the natural history of these lesions, as well as the short- and long-term benefits and risks associated with their repair. The current proposal represents a continuation of the prospective component of the International Study of Unruptured Intracranial Aneurysms (ISUIA). Its primary objectives center around defining the long-term risks of UIA rupture and other UIA natural history outcomes, risk factors associated with these natural history outcomes, long-term outcomes associated with endovascular and surgical repair of these lesions, and predictors of good and poor treatment outcomes. The first two phases of ISUIA have provided substantial and unique information regarding short-term prospective natural history and treatment outcomes, and have established that short-term prospective natural history rupture rates are different than retrospective natural history rupture rates. The current proposal utilizes the large ISUIA cohort of patients established over the past ten years to provide vital long-term clinical information, which would otherwise be unobtainable. The current proposal involves the follow-up of living cases with UIA among the 4,060 cases in the prospective cohort, including 1,692 cases in the unoperated cohort and 2,368 cases in the cohort, which had UIA repair. The additional follow-up will allow the estimation of ten-year hemorrhage rates, and other outcome measures beyond the currently available five-year rates. It will also provide critical information on the long-term durability and effectiveness of endovascular and surgical treatment procedures, which are important in making clinical decisions regarding optimal management. Primary analyses will examine neurologic outcome, specifically fatal and non-fatal intracranial hemorrhagic strokes, secondary to aneurismal rupture and morbidity/mortality following UIA treatment. Secondary analyses will examine other aneurismal complications, such as ischemic stroke and death from all causes, including retreatment, durability of treatment, and functional endpoints, including Rankin, Barthel, and SF36 scores.